Green’s Dictionary of Slang

dadn.1

alsodads[the term flourished, like many similar euph. oaths, in the UK in the late 17C but re-emerged in the 19C in the US, where it remains in many combs., a resurgence poss. helped by the similarity to another taboo word, damned adj.]

[17C+] a euph. for God, often in excls. like by dad! and adad! excl.; also used in a variety of mild verbal oaths (seedad-burn v.) and adjectival oaths (see compounds below).